Transducer structure that operates with acoustic waves

ABSTRACT

For improved conformance of a filter operating with acoustic waves, a transducer structure is provided in which the separations between respectively two adjacent electrode fingers of an inter-digital transducer (IDT v ) vary over the length of the transducer. The variation thereby preferably follows a continuous distribution function. Reactance filters that are provided with such filters in the serial branch show an improved electrical conformance in the transmission range.

BACKGROUND OF THE INVENTION

The invention concerns a transducer structure operating with acousticwaves, particularly for a surface wave filter (called an OFW or also aSAW filter) or an S-BAR filter (bulk acoustic wave resonator).

In filters operating with acoustic waves (particularly in SAW filters)in particular transducer structures operating with acoustic waves (forexample, SAW resonators) are used as impedance elements. Such aresonator is constructed on the surface of a piezoelectric substratefrom metallic electrode structures, and comprises an inter-digitaltransducer with at least two connections that is normally arrangedbetween two reflectors. Known resonators comprise inter-digitaltransducers that are characterized by a uniform finger period and fingerwidth over the entire transducer. Each resonator thereby exhibits whatis known as a resonance frequency and an anti-resonance frequency. Thefrequency position and the intensity of resonance and anti-resonance canbe influenced via variation of the apertures, the finger count and thefinger period. The frequency separation between resonance frequency andanti-resonance frequency, as well as its form, remains the same.

In the reactance filter, the resonators are used as impedance elementsand switched to an arrangement resembling a ladder (i.e., a laddertype). In addition, resonators are arranged in one serial branch and atleast one (preferably, however, a plurality of) parallel branche(s). Theresonance frequency of a resonator in the serial branch is set such thatit approximately corresponds to the anti-resonance frequency of aresonator in the parallel branch. More complex filters, with a pluralityof parallel branches and serially arranged resonators between them, canbe constructed from a plurality of basic components (that respectivelycomprise a parallel and a serial resonator). The interaction of theresonances of the individual resonators generates a desired band-passbehavior of the filter. In addition, the resonance frequencies of theindividual resonators, as well as the intensity of the resonances, aresuitably set. For this purpose, finger periods, finger counts, andapertures of the individual resonators are the known degrees of freedom.

An ideal filter exhibits a good electrical conformance, a good dampingbehavior in the filter attenuation band, and as little insertion loss aspossible in the transmission range. However, its disadvantage is thatthese characteristics can mostly not be simultaneously optimized, suchthat only one suitable combination of characteristics can always beachieved, but not one filter optimal in all characteristics.Particularly in broadband filters that exhibit a relative bandwidth ofmore than 2%, or in filters that are constructed on substrates withlower electro-acoustic coupling (for example, on LiTaO₃ in connectionwith smaller layer thickness or on quartz), an optimization attempt canonly result in non-optimal filters with unsatisfactory characteristics.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide a transducerstructure operating with acoustic waves which comprises at least onefurther degree of freedom in the optimization, and with which, forexample, a reactance filter with improved characteristics can beconstructed.

This object is achieved by a transducer structure operating withacoustic waves with one or more inter-digital transducers (IDT) arrangedbetween reflectors (Ref), at least one IDT comprising: two interlockingelectrode combs having electrode fingers, the finger separations of theIDT, measured between finger centers of each two adjacent electrodefingers, varies over a length of the inter-digital transducer.Advantageous embodiments are described below.

A transducer structure comprises one or more inter-digital transducersarranged between reflectors. These again comprise electrode fingersconnected with busbars that interlock like combs. In contrast to knowntransducers, in the inter-digital transducers of the transducerstructures according to the invention, the distance between the fingercenters (i.e., the finger period) per two adjacent electrode fingers isnot constant; rather it varies over the entire length of theinter-digital transducer.

It has been shown that a better electric conformance can be achieved ina resonator-filter environment with such a variation of the fingerperiod. The filter can be a DMS filter, a TCF filter, or a reactancefilter. In particular, the reflection at the input or, respectively,output of the filter can be minimized with a transducer structure withvarying finger period. With the invention, for example, the voltagestanding wave ratio (VSWR) can be reduced in a reactance filter used inthe high-frequency range. Finally, with resonators according to theinvention, reactance filters can be constructed that exhibit an improvedpassband and, in particular, an improved insertion loss.

In an embodiment of the invention, the variation of the fingerseparations can be undertaken such that the precise values for thefinger separations (finger periods) applied over the length of theinter-digital transducer come to lie on a curve corresponding to acontinuous function. The precise values for the finger separations at apoint x thus correspond to the value of the continuous function scannedat the point x. Preferably, a quasi-continuous function is selected.Such a function exhibits no discontinuities.

In another embodiment, a further advantageous variation of the fingerseparations is achieved when the cited spreading of the fingerseparations over the length of the transducer follows a function that issymmetric around an axis perpendicular to the direction of wavepropagation, whereby the axis is preferably located in proximity to thecenter of the transducer. A function is preferably selected thatexhibits a maximum at the axis of reflection.

A simple variation of the finger separations over the length of theinter-digital transducer follows a linear function in which the fingerseparations linearly increase or, respectively, decrease in a direction.The spreading of the finger separations can be such that the increaseensues from one end to the other end of the inter-digital transducer, orthat the increase or decrease ensues up to the axis of reflection andthereafter once again decreases or, respectively, increases.

It is additionally possible in an embodiment to spread the fingerseparations over the length of the inter-digital transducer according toa nonlinear function, for example, according to a parabolic function.This can also be selected such that it reaches an extreme value in themiddle of the transducer that represents either a minimum or a maximum.

The finger separations may be varied around an average value up to amaximum of +/−2.5%, such that a maximal difference of 5% results betweentwo finger separations. Typically, inter-digital transducers accordingto the invention exhibit a maximum difference of 2 to 3%, for example of3%. Advantageous improvements are thereby already achieved in reactancefilters with smaller differences than, for example, DMS filters. Thelatter can fully utilize the stated variation width and exhibit adifference up to 5%.

A further variation possibility of an embodiment of the inventiveresonator arises via an additional variation of the finger widths of theelectrode fingers viewed over the length of the inter-digitaltransducer. This variation also preferably follows a continuousfunction. The variation of the finger widths can be undertaken such thatthe metallization proportion remains constant over the length of thetransducer. However, other variations are also possible in which themetallization proportion over the length of the transducer continuallyincreases or decreases, or in that the metallization proportion followsthe corresponding distribution function of the finger separations, asthe case may be, a function reflected in the center of the transducer.

An embodiment of the inventive resonator develops with particularadvantages in a reactance filter, in which it is arranged either in aparallel branch or, preferably, a serial branch. Since the form of theresonance of the resonators is changed with the inventive variation offinger widths and/or metallization proportion, particularly the passbandis influenced. Since this is substantially determined via the resonatorsarranged in the serial branch, a maximum result is achieved withinventive resonators in the serial branch. In this manner, theelectrical characteristics of the reactance filter can be changed andoptimized.

With an embodiment of the invention, a spreading of the resonance can beachieved for the individual resonator, whereby simultaneously the shapeof the anti-resonance remains unchanged. In this manner, a reactancefilter can be achieved which exhibits an increased bandwidth without theother characteristics of the filter simultaneously deteriorating. Withan embodiment of the invention, reactance filters may be achieved viasuitable selected variations of the finger separations and finger widthsthat, given the same bandwidth, overall show an improved electricalconformance in the transmission range of the filter.

A reactance filter with an individual inventive resonator in the serialbranch already shows an improved electrical behavior. However, allresonators arranged in the serial branch are preferably provided withinventively varying finger separations and/or varying finger widths. Inembodiments of the invention, the variation widths of the fingerseparations can thereby fluctuate between 0.1 and 3% around the medianfinger separation. In this manner, reactance filters are achieved thatexhibit an increased relative bandwidth of more than 2%.

Particularly advantageous are inventive resonators or, respectively,filters produced from them, constructed with poorer electro-acousticcoupling, for example, on lithium tantalate with low layer thickness.Under such conditions, a better electrical conformance is achieved viaan improved electro-acoustic behavior.

DESCRIPTION OF THE DRAWINGS

The invention is more closely explained with reference to theembodiments illustrated in the following figures.

FIG. 1 is a pictorial/schematic diagram showing a known resonator;

FIG. 2 is a schematic diagram showing a known structure for a reactancefilter;

FIG. 3 is a pictorial diagram showing an embodiment of the inventiveresonator;

FIGS. 4–6 are graphs showing inventive functions for spreading of thefinger widths over the length of the transducer;

FIG. 7 is a graph comparing the voltage standing wave ratio of the knownand inventive reactance filters;

FIG. 8 is a graph comparing the transmission range of an inventivereactance filter with that of a known reactance filter; and

FIG. 9 is a pictorial diagram illustrating an IDT having varyingdistance and width of electrode fingers over the length of the IDT.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A resonator operating with acoustic waves in shown in FIG. 1. Themetallic electrode structures (for example, comprising aluminum, analuminum alloy, or a multi-layer structure comprising an aluminum layer)are applied to a piezoelectric substrate. The resonator consists of aninter-digital transducer IDT that is arranged between two reflectorsRef. Each inter-digital transducer IDT comprises of two telescopedcomb-like electrodes that are connected with connections T1 and T2. Theelectrode fingers of the inter-digital transducer of known resonatorsexhibit a uniform electrode finger separation P that typically isdefined as the distance between the finger centers of two adjacentelectrode fingers. The circuit symbol for such a resonator R is shown inthe right half of the image. Known possibilities to adjust theelectrical properties of such a resonator R exist in the variation ofthe aperture A, the number N of the electrode fingers, the fingerseparation P of the electrode fingers, and the metallization proportionη which, viewed in the direction of wave propagation, defines theproportion of the metallized surface to the entire surface.

FIG. 2 shows an example for a reactance filter that is arranged in theform of a branch switch or, respectively, a ladder-type structure. Thiscomprises a serial branch, in which serial resonators R_(s) arearranged, between an input IN and an output OUT. Beside this, twoparallel branches are shown in the figure in which, respectively, oneparallel resonator R_(p) is arranged. Typically, the parallel branch isconnected to ground. However, it is also possible to symmetricallyoperate reactance filters. The resonators feature an impedance behaviorwhich, given a resonance frequency, exhibit a minimal impedance;however, given an anti-resonance frequency, they exhibit a maximalimpedance. Via suitable modulation of the resonance frequencies andanti-resonance frequencies of the resonators in the serial and in theparallel branches, the reactance filter fashions a passband.

FIG. 3 shows an embodiment of the inventive inter-digital transducerIDT_(v) with finger separation varying over the length of theinter-digital transducer. The spreading of the finger separations overthe length of the transducer thereby follows a function, as it isspecified in FIG. 4, for example. In an inventive transducer, the fingerseparations P spread over the number of fingers n lie on a preferablycontinuous function (in FIG. 4, for example, on a linear function). Thefinger separation P thereby falls over the length of the transducer froma maximum finger separation Pmax, culminating at a minimal fingerseparation Pmin. However, spreadings of the finger separations are alsopossible as they are shown in FIGS. 5 and 6, for example. In FIG. 5, alinear spreading over the length of the transducer is likewise shown,whereby the overall distribution function is composed of two linearsub-functions that are arranged symmetric to one another in terms of anaxis of reflection lying in the region of a median electrode finger Nmand vertical to the direction of wave propagation X.

As noted above, and as illustrated by FIG. 9, further variationaccording to an embodiment of the inventive resonator arises via anadditional variation of the finger widths of the electrode fingersviewed over the length of the inter-digital transducer. This variationalso preferably follows a continuous function. The variation of thefinger widths can be undertaken such that the metallization proportionremains constant over the length of the transducer. However, othervariations are also possible in which the metallization proportion overthe length of the transducer continually increases or decreases, or inthat the metallization proportion follows the corresponding distributionfunction of the finger separations, as the case may be, a functionreflected in the center of the transducer.

FIG. 6 shows a spreading of the finger separations P that corresponds toa parabolic function whose maximum is placed here in the region of thetransducer center.

In addition to the spreadings of the finger separations shown in FIGS. 4through 6, further functions are possible and advantageous that lead toan inventive resonator that yields improved characteristics forreactance filters manufactured therefrom. It is not required that thedistribution function show a symmetry behavior as shown. However, withregard to losses, it is mostly advantageous that the finger separationcontinually changes, that the distribution function thus exhibits nodiscontinuities.

As an exemplary embodiment, an inventive resonator which comprises aninter-digital transducer IDT_(v) with linear varying finger separation(as shown in FIG. 3) is used for production of a reactance filter. Inaddition, resonators as shown in FIG. 2 are interconnected to areactance filter. The serial resonators R_(s) 1 through R_(s) 3 arefashioned via inventive resonators, while the parallel resonators areconventional resonators (for example, as shown in FIG. 1).

A suitable application for the invention arises, for example, in a2-in-1 filter used for mobile telephones, in which a 1 GHz reactancefilter and a 2 GHz reactance filter are combined on a single substratewith a uniform metallization layer thickness of, for example, 230 nm.The layer thickness of 230 nm is approximately 40% less than an optimaland typical metallization layer thickness used in 1 GHz filters. Thisleads, in the 1 GHz filter, to a clear degradation of the electricalconformance, as a consequence of which an in increased signal reflectionarises at the filter input and output. This manifests in a voltagestanding wave ratio VSWR of approximately 3.6, which in turn requires acorrespondingly high damping of the transmitted signal in thetransmission range. This unnecessarily high insertion loss generateslosses whose avoidance is sought.

In FIG. 7, the gradient M2 for the voltage standing wave ratio VSWR ofthe specified inventive reactance filter is shown, which is contrastedto the gradient M1 of a corresponding reactance filter with conventionalresonators. The gradient M1 of the reactance filter constructed withknown resonators shows a maximum voltage standing wave ratio ofapproximately 3.6 in the transmission range, which is indicated in theupper region of the figure by a rectangle. The reactance filter providedwith inventive resonators in the serial branch in contrast produces thegradient M2, which shows a clearly improved voltage standing wave ratioof approximately 2.3.

The shown transmission behavior likewise determined via a comparisonmeasurement using the function S21 is reproduced in FIG. 8. The curve D1for the damping of the known reactance filter runs below the curve D2for the damping behavior of the inventive reactance filter over almostthe entire passband. It thus shows that the electrical conformance canbe substantially improved with the invention, which in particular showsin the presented lower insertion loss of the inventive reactance filter.

In an embodiment for further improvement of inventive reactance filters,the metallization proportion can additionally be still set such that itis constant over the transducer, or varies corresponding to adistribution function. An optimal distribution function for the fingerseparations can be determined with the aid of an automatic optimizationthat can be implemented with software developed on the basis ofelectro-acoustic models. In each case, the invention shows a simple wayto improve in a simple manner a poorly adapting filter, wherebypreviously required additional adapting elements can be dispensed with.

For the purposes of promoting an understanding of the principles of theinvention, reference has been made to the preferred embodimentsillustrated in the drawings, and specific language has been used todescribe these embodiments. However, no limitation of the scope of theinvention is intended by this specific language, and the inventionshould be construed to encompass all embodiments that would normallyoccur to one of ordinary skill in the art. The particularimplementations shown and described herein are illustrative examples ofthe invention and are not intended to otherwise limit the scope of theinvention in any way. For the sake of brevity, conventional electronicsand other functional aspects of the systems (and components of theindividual operating components of the systems) may not be described indetail. Furthermore, the connecting lines, or connectors shown in thevarious figures presented are intended to represent exemplary functionalrelationships and/or physical or logical couplings between the variouselements. It should be noted that many alternative or additionalfunctional relationships, physical connections or logical connectionsmay be present in a practical device. Moreover, no item or component isessential to the practice of the invention unless the element isspecifically described as “essential” or “critical”. Numerousmodifications and adaptations will be readily apparent to those skilledin this art without departing from the spirit and scope of the presentinvention.

1. A reflector-based resonator structure operating with acoustic waveswith one or more inter-digital transducers (IDT) arranged betweenreflectors (Ref), at least one IDT comprising: two interlockingelectrode combs having electrode fingers, the finger separations of theIDT, measured between finger centers of each two adjacent electrodefingers, varies over a length of the inter-digital transducer accordingto a continuous function and wherein a difference between theseparations of adjacent electrode fingers amounts to a maximum of 5%viewed over the total length of the IDT.
 2. The resonator structureaccording to claim 1, wherein the variation of the finger separationscorresponds to a function that, in proximity to a center, is symmetricaround an axis perpendicular to a direction of wave propagation.
 3. Theresonator structure according to claim 1, wherein a spreading of thefinger separations over the length of the IDT is conformed to a linearfunction.
 4. The resonator structure according to claim 1, wherein aspreading of the finger separations over the length of the IDT isconformed to a parabolic function.
 5. The resonator transducer structureaccording to claim 1, wherein the difference amounts to a maximum of2–3%.
 6. The resonator structure according to claim 1, whereinadditionally widths of the electrode fingers vary over the length of theIDT, and their separation is conformed to a continuous function.
 7. Theresonator structure according to claim 1, wherein the finger separationsare spread over the length of the IDT such that the finger separationsin the center of the transducer are the largest.
 8. The resonatorstructure according to claim 1, wherein the IDT is arranged in areactance filter which comprises a serial and at least one parallelbranch, in which respectively at least one resonator is arranged.
 9. Theresonator structure according to claim 8, wherein the IDT is arranged inthe serial branch of the reactance filter.
 10. The resonator structureaccording to claim 9, wherein electrode fingers of all resonatorsarranged in the serial branch exhibit at least one of varying fingerseparations and varying finger widths.
 11. The resonator structureaccording to claim 1, wherein the variation widths of the fingerseparations, with regards to an average finger separation, amount to+/−0.1% to 2.5%.
 12. A reactance filter comprising a resonator structureaccording to claim 1, the reactance filter having a large relativebandwidth delta f_(rel), in that 2%<delta f_(rel)<5% is valid.
 13. Afilter on a lithium tantalate (LT) substrate or on a substrate having apoorer electro-acoustic coupling than LT comprising a resonatorstructure according to claim
 1. 14. The resonator structure according toclaim 1, wherein the variance according to a continuous function ensuesover an entire length of the IDT.
 15. The resonator structure accordingto claim 14, wherein the distance between any two adjacent pairs offingers of the IDT is not equal.